Since 1996, farmers worldwide have planted more than 1 billion acres (400 million hectares) of genetically modified corn and cotton that produce insecticidal proteins from the bacterium Bacillus thuringiensis, or Bt for short.

Bt proteins, used for decades in sprays by organic farmers, kill some devastating pests but are considered environmentally friendly and harmless to people. However, some scientists feared that widespread use of these proteins in genetically modified crops would spur rapid evolution of resistance in pests.

A team of experts at the University of Arizona has taken stock to address this concern and to figure out why pests became resistant quickly in some cases, but not others.

Bruce Tabashnik and Yves Carrière in the department of entomology at the College of Agriculture and Life Sciences, together with visiting scholar Thierry Brévault from the Center for Agricultural Research for Development, or CIRAD, in France, scrutinized the available field and laboratory data to test predictions about resistance. Their results are published in the journalNature Biotechnology.

"When Bt crops were first introduced, the main question was how quickly would pests adapt and evolve resistance," said Tabashnik, head of the UA department of entomology who led the study. "And no one really knew; we were just guessing."

"Now, with 1 billion acres of these crops planted over the past 16 years, and with the data accumulated over that period, we have a better scientific understanding of how fast the insects evolve resistance and why."

Analyzing data from 77 studies of 13 pest species in eight countries on five continents, the researchers found well-documented cases of field-evolved resistance to Bt crops in five major pests as of 2010, compared with only one such case in 2005. Three of the five cases are in the United States, where farmers have planted about half of the world's Bt crop acreage.

Their report indicates that in the worst cases, resistance evolved in two to three years, but in the best cases, effectiveness of Bt crops has been sustained more than 15 years.

According to the paper, both the best and worst outcomes correspond with predictions from evolutionary principles.

"The factors we found to favor sustained efficacy of Bt crops are in line with what we would expect based on evolutionary theory," said Carrière, explaining that conditions are most favorable if resistance genes are initially rare in pest populations; inheritance of resistance is recessive – meaning insects survive on Bt plants only if have two copies of a resistance gene, one from each parent – and abundant refuges are present. Refuges consist of standard, non-Bt plants that pests can eat without ingesting Bt toxins.

"Computer models showed that refuges should be especially good for delaying resistance when inheritance of resistance in the pest is recessive," explained Carrière.

Planting refuges near Bt crops reduces the chances that two resistant insects will mate with each other, making it more likely they will breed with a susceptible mate, yielding offspring that are killed by the Bt crop. The value of refuges has been controversial, and in recent years, the EPA has relaxed its requirements for planting refuges in the U.S.